Dimethyl hydrogen phosphite (DMHP) has been recently tested by the National Toxicology Program for chronic toxity and carcinogenicity in F344/N rats and B6C3Fl mice. DMHP was found to be carcinogenic to rat lungs but had no effect on mice. This study is designed to investigate various factors, related to pharmacokinetics, metabolism and disposition which may be involved in species sensitivity to DMHP.